Architectural Record BE - Building Enclosure

Building Materials Matter

Life cycle view supports informed choices, contributes to sustainable design
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Initial Embodied Impacts: From Extraction to Construction

The impact of materials from extraction or harvesting through manufacturing, transportation, and construction are considered initial embodied impacts. They are distinct from operational impacts, which result from a building’s operation, and from recurring embodied impacts, which relate to the durability of building materials, components, and systems; how well they’re maintained; and the service life of the building.

Building materials tend to have the greatest impact from extraction through manufacturing. Within an LCA, this is also where wood’s advantages are most evident.

Wood products store carbon. In the case of wood buildings, the carbon is kept out of the atmosphere for the lifetime of the structure—or longer if the wood is reclaimed and reused or manufactured into other products.

Wood products store carbon. In the case of wood buildings, the carbon is kept out of the atmosphere for the lifetime of the structure—or longer if the wood is reclaimed and reused or manufactured into other products.

Raw Materials

The life cycle of building products typically starts with the extraction of raw resources such as timber, iron ore, limestone, and aggregates. The collection of data starts here, with the tracking of energy use and emissions to air, water, and land per unit of resource.

Wood’s impacts during this phase are relatively low compared to concrete and steel, which are made from substances that must be mined and heated to extremely high temperatures.5

A typical concrete mix is about 10 to 15 percent cement, 60 to 75 percent aggregate, and 15 to 20 percent water, though proportions change to achieve different requirements for strength and flexibility. While most of concrete’s ingredients are themselves manufactured products or mined materials, it’s the cement in concrete that has the highest embodied energy.6 According to the U.S. Energy Information Administration, the cement industry is the most energy-intensive of all manufacturing industries. Cement is also unique in its heavy reliance on coal and petroleum coke.7

A major ingredient needed for cement is limestone, which is found in abundance in many places in the world. In most cases, limestone is blasted from surface mines and removed in large blocks to a crusher, mixed with other raw materials, and transferred to a rotating furnace where it is heated to about 2,700 degrees Fahrenheit in order for the materials to coalesce. The mixture is cooled and ground to fine powder (cement), which is transported to its destination by truck, rail, or ship. Fly ash, a byproduct of coal burning, can be substituted for some of the cement, as can a variety of other ingredients, with associated reductions in carbon footprint.

Steel is an alloy consisting mainly of iron and has a carbon content between 0.2 percent and 2.1 percent by weight, depending on grade. Steel’s main ingredient is iron ore, which must be extracted through open pit mining and heated to extremely high temperatures. In surface mines, ground is removed from large areas to expose the ore. Ore is then crushed, sorted and transported by train or ship to the blast furnace where the iron is heated to 3,000 degrees Fahrenheit, usually with charcoal or coke, and charged with the ore and limestone. The molten iron drains off, and iron ingots are formed. This pig iron, as the ingots are called, is the basis for steel.

For both concrete and steel there are environmental consequences from open pit mining, and from the fossil fuels used to process the raw materials. However, both industries continue making strides to lighten their environmental footprint.

Manufacturing

While the manufacturing stage typically accounts for the largest proportion of embodied energy and emissions associated with the life cycle of a building product, it is also an area where wood consistently outperforms steel and concrete.10

Opportunities for lessening the impacts of open pit mining include reducing the size of the mining area, minimizing waste, helping to maintain biodiversity by transplanting or culturing endangered plants found on-site, and planning mines around existing infrastructure.<sup>11</sup>

Photo courtesy of Sergey Zavalnyuk, Dreamstime

Opportunities for lessening the impacts of open pit mining include reducing the size of the mining area, minimizing waste, helping to maintain biodiversity by transplanting or culturing endangered plants found on-site, and planning mines around existing infrastructure.11

The process at a lumber mill is relatively straightforward. Bark is removed; logs are sawn; trimmed to produce smooth, parallel edges; cut to square and precise lengths; dried; and then planed, grade-stamped, and packaged.

For mass timber products, which have structural performance characteristics that allow them to compete with steel or concrete in many applications, the process is more involved. For glue-laminated timber, for example, wood laminations are bonded with durable, moisture-resistant adhesives. For cross laminated timber (CLT), several layers of kiln-dried lumber boards stacked in alternating directions are bonded with structural adhesives to form a solid, straight, rectangular panel. LCAs of mass timber products are discussed later in this course (see Toward a Sustainable Future).

Throughout the years, the lumber industry has set its sights on getting the most out of every tree harvested and brought to a mill. According to a report on wood utilization, “The term ‘waste’ is largely obsolete in the context of today’s North American forest-products industry. Logs brought to U.S. and Canadian sawmills and other wood-product manufacturing centers are converted almost totally to useful products.”12

This achievement can be attributed to state-of-the-art sawmilling that maximizes the quality and quantity of boards that can be cut from a tree, combined with further processing of fiber that is unsuitable for lumber production into composite products such as oriented strand board (OSB) or fiber boards and paper.

Producing concrete requires mixing cement, which has already been manufactured, with mined aggregates. Ready-mixed concrete is the most common form of concrete, accounting for up to 75 percent of the material made today. This is concrete that is “batched” from a central plant. Each batch is customized to the requirements of the specific job, and is usually delivered to site in cement-mixer trucks.

Iron smelted from ore contains more carbon than is desirable. To become steel, the iron must be melted, again at extremely high temperatures, and reprocessed to reduce the carbon, and to remove silica, phosphorous, and sulfur, which weaken the steel.

There are two main technologies for producing steel in the United States. One involves a Basic Oxygen Furnace (BOF), in which high-purity oxygen blows through the molten pig iron, lowering carbon levels and those of other impurities. Alloys are added at this time to create the desired properties of the steel product. The other approach involves “mini mills” that use Electric Arc Furnaces (EAF) to produce steel from metal scrap.

Using wood products avoids the carbon emissions inherent in the industrial processes of concrete or steel products. While the concrete and steel industries are primarily powered by fossil fuels, many lumber companies use woody biomass (e.g., sawmill residues such as bark and sawdust) to fuel their operations. Dovetail Partners Inc., which provides information about the impacts and trade-offs of environmental decisions, calls the North American lumber industry 50 to 60 percent energy self-sufficient overall.

To manufacture CLT, several layers of kiln-dried lumber boards are stacked in alternating directions and bonded with structural adhesives to form solid, straight, rectangular panels.

Photo courtesy of naturallywood.com

To manufacture CLT, several layers of kiln-dried lumber boards are stacked in alternating directions and bonded with structural adhesives to form solid, straight, rectangular panels.

Construction

According to the Athena Sustainable Materials Institute, which specializes in LCA, the on-site construction stage is similar to an additional manufacturing step where individual products, components, and sub-assemblies come together in the manufacture of a building.

Although transportation may comprise a significant portion of the impacts at this stage, the prescriptive approach that says all materials should be produced locally may not yield the best environmental outcome. harvested and brought to a mill. According to a report on wood utilization, “The term ‘waste’ is largely obsolete in the context of today’s North American forest-products industry. Logs brought to U.S. and Canadian sawmills and other wood-product manufacturing centers are converted almost totally to useful products.”12

This achievement can be attributed to state-of-the-art sawmilling that maximizes the quality and quantity of boards that can be cut from a tree, combined with further processing of fiber that is unsuitable for lumber production into composite products such as oriented strand board (OSB) or fiber boards and paper.

Producing concrete requires mixing cement, which has already been manufactured, with mined aggregates. Ready-mixed concrete is the most common form of concrete, accounting for up to 75 percent of the material made today. This is concrete that is “batched” from a central plant. Each batch is customized to the requirements of the specific job, and is usually delivered to site in cement-mixer trucks.

Iron smelted from ore contains more carbon than is desirable. To become steel, the iron must be melted, again at extremely high temperatures, and reprocessed to reduce the carbon, and to remove silica, phosphorous, and sulfur, which weaken the steel.

There are two main technologies for producing steel in the United States. One involves a Basic Oxygen Furnace (BOF), in which high-purity oxygen blows through the molten pig iron, lowering carbon levels and those of other impurities. Alloys are added at this time to create the desired properties of the steel product. The other approach involves “mini mills” that use Electric Arc Furnaces (EAF) to produce steel from metal scrap.

Using wood products avoids the carbon emissions inherent in the industrial processes of concrete or steel products. While the concrete and steel industries are primarily powered by fossil fuels, many lumber companies use woody biomass (e.g., sawmill residues such as bark and sawdust) to fuel their operations. Dovetail Partners Inc., which provides information about the impacts and trade-offs of environmental decisions, calls the North American lumber industry 50 to 60 percent energy self-sufficient overall.

 

[ Page 3 of 6 ]         
Originally published in Architectural Record

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